Obesity rate has been on the rise in the United States over the past decades. While life style change in diet and physical activities is recognized as the primary cause of obesity, there are other contributing factors as well. Many common drugs prescribed to millions of people each year have been clinically linked to significant weight gain as a result of undesired side effect (referred to as obesogenic effect), but the underlying pharmacological mechanisms are poorly understood. Medication-induced obesity could lead to other health risks including diabetes and cardiovascular diseases, as well as a risk of adverse effect from discontinuing the medication. The lack of understanding of how different medications can cause weight gain makes it difficult to prevent or counteract this side effect. To help address this knowledge gap, this study proposes to examine 14 drugs of known obesogenic effects for potential direct disruption of lipogenesis and lipolysis, the two opposing events in regulating adipose tissue homeostasis. The adipose tissue is composed of predominantly adipocytes (fat cells), in addition to preadipocytes (precursors of adipocytes), human mesenchymal stem cells (hMSCs) and a few other cell types including macrophages and endothelial cells. Accumulation of fat results from adipogenic differentiation of hMSCs, maturation of preadipocytes into adipocytes or continuous accumulation of fat in adipocytes, whereas loss of fat results from the breakdown of fat into glycerol and fatty acids in adipocytes. Weight gain could result from accumulation of fat or decrease in fat reduction. Specifically, the following 4 aims are proposed to examine the direct effect of selected obesogenic drugs on adipose tissue: Aim 1, determine how each drug singularly or in combination with other treatments could affect the cell fate determination of hMSCs to become fat cells or bone cells; Aim 2, examine how each drug singularly or in combination with other treatments could affect the differentiation of preadipocytes into adipocytes under either acute or chronic exposures. Two subtypes of preadipocytes from the same donor, intra-abdominal and subcutaneous, will be tested in order to assess potential differential response of cells from different fat depots; Aim 3, examine the potential effect of each drug on the accumulation and lipolytic rate of fat in adipocytes under acute or chronic exposures; And Aim 4, explore the molecular mechanisms underlying the identified in vitro obesogenic effect of drugs in the previous 3 aims. This proposed study would help to gain knowledge about the potential direct actions of known obesogenic drugs on adipose homeostasis and potentially shed new light on the molecular mechanisms underlying the regulation of adipose homeostasis. Knowledge obtained from this study will help clinical and public health professionals provide more informed care of their patients and develop treatment strategies for preventing drug associated weight gain. Furthermore, it will help establish prototypes of testing modules for predicting adipogenic toxicity of a wide range of existing drugs in the market, drugs in development, as well as environmental chemicals.